test_exception.rb: ignore SystemStackError

* test/ruby/test_exception.rb (test_machine_stackoverflow): ignore
  propagated SystemStackError, which has not bee rescued in the
  child process by unknown reason.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44124 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_exception.rb

@@ -485,6 +485,7 @@ end.join
       h[:a].call
     }
     SRC
+  rescue SystemStackError
   end
 
   def test_cause